Process for preparing composite sheet material



United States Patent 3,516,883 PROCESS FOR PREPARING COMPOSITE SHEET MATERIAL Lee R. Harper, Media, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Feb. 1, 1967, Ser. No. 613,121 Int. Cl. B32b 3/00 U.S. Cl. 156-77 15 Claims ABSTRACT OF THE DISCLOSURE Leather-like sheet materials and the like are prepared by applying to a suitable substrate a layer of a liquid polymeric coating composition, exposing said layer to an air stream having sufiicient velocity and humidity to cause at least the uppermost portion of the layer to coagulate, and then subjecting the polymeric layer to a bathing and drying procedure whereby it becomes a solidified microporous structure.

This invention relates to a process for preparing sheet material; the invention especially concerns the preparation of moisture-permeable sheet material, with particular emphasis on the preparation of leather-like composite sheet material useful for the manufacture of shoe-uppers.

In a broad sense, the process of this invention comprises:

(A) applying to a suitable substrate a layer of a liquid polymeric composition which is comprised of a polymer dispersed in a volatile liquid comprising a volatile organic solvent, and which layer is capable of being solidified in situ to form a microporous structure when applied to a substrate and then subjected to steps B, C and D below,

(B) exposing said layer of liquid composition to an air stream having a velocity of at least feet per minute and a relative humidity of at least 16% (when measured at a dry bulb temperature of about 27 C.) until said layer has absorbed enough water to cause at least the uppermost portion of the layer to coagulate,

(C) bathing said layer of composition with a volatile bathing liquid adapted to extract the volatile organic solvent from said composition until substantially all of said solvent is extracted, and

(D) drying the resulting sheet material, steps B, C and D causing said composition to solidify in situ and become a microporous moisture-permeable structure.

In some preferred embodiments of the invention, the substrate to which the composition is applied is a porous fibrous substrate, and the microporous structure resulting from the novel process forms an adherent coating on the substrate (e.g., as illustrated in Example 1). It is also possible to use a so-called non-adherent porous fibrous substrate; that is, a substrate which has been treated prior to step (A) in a manner which permits the subsequent removal of the microporous structure in the form of an unsupported film (e.g., as illustrated in Example 2).

A preferred type of porous fibrous substrate is a fabric; however, other known porous fibrous sheet materials can also be used as the substrate, for example paper, leather, man-made suede materials and webs formed by paper making techniques from fibrids or mixtures of fibrids and fibers. The fabric can be a woven fabric or a nonwoven fabric of any natural fibers or any synthetic fibers or blends thereof. Also useful are composite fabrics formed by assembling woven fabrics in a face-to-face contact with nonwoven fabrics. One very useful fabric is a nonwoven fabric containing a solidified polymeric impregnant having a porous moisture-permeable structure;

3,516,883 Patented June 23, 1970 ice such a nonwoven fabric can be prepared for example by the procedure described in Example 1 in U.S. Pat. 3,067,483 issued Dec. 11, 1962 to J. L. Hollowell.

A fabric or other suitable substrate is used which is made of materials that do not dissolve to any harmful extent in any solvent present in the liquid composition applied in step (A) or in bathing liquid used in step (C) of the process as described above.

In other embodiments of the invention, the substrate to which the composition is applied is a nonporous substrate, preferably one from which the microporous structure can be removed as an unsupported film; useful substrates of this type include polymer coated fabrics, glass sheets, metal strips and impervious polymeric sheet materials which do not strongly adhere to the polymeric composition applied in the novel process.

An especially useful liquid polymeric composition to apply to the substrate in the practice of this invention is a substantially colloidal polymeric dispersion which has been prepared by adding a nonsolvent to a polymer solution according to the method described in claim 1 of U.S. Pat. 3,100,721 issued Aug. 13, 1963 to E. K. Holden.

Another example of a useful liquid polymeric composition is the coating composition employed in Example 1 of U.S. Pat. 3,238,055 issued Mar. 1, 1966 to E. P. Brightwell.

In various preferred embodiments of the invention, the liquid polymeric composition to be applied to the substrate is prepared by adding water to the polymer solution in an amount sufiicient to provide the composition with a water content of about 3.7-4.5 based on the total weight of the resulting composition.

In step (B) of the process, the layer of liquid polymeric composition which has been applied to the substrate is exposed to an air stream having the required relative humidity and having a velocity of at least 10 feet per minute, preferably about 50-300 feet per minute, and still more preferably about -200 feet per minute. Thus, the most beneficial results are obtained when the air stream has a relatively high velocity within a specific range. The layer of composition is exposed to the air stream for a period of time suflicient to permit said layer to absorb enough water to cause at least the uppermost portion of the layer to coagulate. The velocity of the air stream is preferably high enough to permit the layer of composition to absorb the required amount of water rapidly and uniformly enough to produce a substantially blemish-free coating structure of intercommunicating micropores having a substantially uniform pore size. When the velocity is too low, the microporous coating (or film) of the product tends to contain blemishes and have a pattern resembling the footprints of a chicken, these blemishes are sometimes referred to in the art as surface tracks. However, if the air stream velocity is too high, there is a tendency to obtain noticeable or harmful roughening (e.g., ripples) in the surface of the product; also there is sometimes an undue reduction in the bond strength between the layers of the composite product when using a porous fibrous substrate. In many of the preferred embodiments of this invention, an air stream velocity of about -160 feet per minute is employed for optimum results.

The air stream preferably has a relative humidity of about 30-50% (when measured at a dry bulb temperature of about 27 C.), and still more preferably about 35-40%; the latter range is especially desirable when the velocity is within the referred range of about 100-200 feet per minute. A relative humidity is employed which permits the layer of composition to absorb the required amount of water rapidly and uniformly, thereby producing a microporous coating structure which is substantially free of collapsed cells. However the relative humidity 3 must not be so high as to cause the coating structure to have such large pores that the product has unsatisfactory abrasion resistance and dyeing characteristics.

A person skilled in the art, after reading the present disclosure, will have little difficulty in selecting the air stream velocity, temperature and Water vapor content that gives the most satisfactory combination of properties in the final product for the particular liquid polymeric compsition and duration of air stream exposure being employed in the practice of this invention.

The temperature of the layer of polymeric composition at the beginning of step (B) is preferably about 20-35 C., and the temperature of the air stream is preferably about 0.55.0 degrees lower than the temperature of the layer of composition. If the temperature of the air stream is too much above the temperature of the layer of liquid composition, there is a tendency to obtain a product having the surface track blemishes mentioned above.

Step (B) is conveniently and efficiently carried out by passing the coated substrate through a rapidly moving air stream in the manner illustrated in Example 1; or by passing the coated substrate through a plenum chamber wherein the air stream passes horizontally across the surface of the liquid composition layer as the air stream enters the chamber through numerous evenly-spaced holes in a box-like wall on one side of the chamber and leaves the chamber through a similar wall on the opposite side of the chamber. Step (B) can also be carried out by any other known method of directing an air stream across or against the surface of a sheet material so that all portions of the surface receive substantially the same amount of exposure to the air stream. For example, the air stream can be conducted towards the layer of composition through one or more air ducts of suitable size and shape.

The bathing liquid employed in step (C) of the process is preferably water or a mixture of water and a watermiscible volatile organic liquid (e.g., dimethylformamide). The bathing liquid is volatile in the sense that it can be evaporated from the sheet material under drying conditions which cause no undue damage to the fiber and polymer components. The bathing liquid also is a coagulant for the liquid polymeric composition employed in preparing the sheet material; thus, the bathing liquid is adapted to coagulate any of the polymeric compositions which remains uncoagulated after step (B) when the bathing liquid comes in contact with the composition.

Leather-like sheet materials can be prepared in accordance with this invention in which substantially all of the pores in the microporous coating are below 20 microns in diameter; the pores are often less than microns in diameter. The pores communicate with one another.

The process of the invention has beneficial utility in the preparation of moisture-permeable sheet materials, including leather-like sheet materials useful for the manufacture of shoe-uppers, luggage, upholstery and the like. The process makes it possible to provide a microporous coating wherein the pore size is not only extremely small but also surprisingly uniform. Products are obtainable by this process having a desirable combination of properties including good abrasion resistance, breathability, blemish-free surface and the capacity to be dyed to uniform color. Since the process permits the desirable microporous structure of the coating to be formed with great speed and efiiciency, it has important economical advantages for the large-scale production of high quality leather-like products. Unsupported microporous films can be made by the process of this invention which are useful in such applications as filters and surgical bandages, these films can also be bonded to fabrics and the like for the manufacture of leather-like sheets and many other useful composite articles.

The examples which follow are given for the purpose of illustrating the invention. All quantities shown are on a weight basis unless otherwise indicated.

4 EXAMPLE 1 A moisture-permeable leather-like sheet material having a substantially blemish-free surface, the capacity to be dyed to a uniform color, and good abrasion resistance and breatha'bility is prepared by (1) providing a woven blended-fiber fabric of a type commonly used in making mens shirts, whichfabric (a) has been made from yarns composed of 65% polyester fibers (polyethylene terephthalate) and 35% cotton fibers, (b) has a thickness of 5.5 mils, a Weight of 2.3 ounces per square yard, a thread count (of the as-woven fabric) of '96 x 96 (warp x fill), a width of 14 inches and a length of 100 yards, (c) has "been scoured until free of processing lubricant, sizing and other foreign matter, (d) has been bleached and double-singed on each side, and (c) has been stretched slightly while wet and then dried to a moisture content of about 0.2% in a conventional tenter-frame oven before being wound up on a storage roll;

(2) providing a nonwoven fabric which (a) has been made from polyester fibers, b) contains as a solidified polymeric impregnant a polyurethane elastomer having a porous moisture-permeable structure, (c) has been prepared in accordance with the teaching of Example 1 of U.S. Pat. 3,067,483 (mentioned above), but greater in thickness, (d) has a thickness of 60 mils, a density of 0.38 gram per cubic centimeter, a width of 14 inches and a length of 100 yards, and (e) has been dried to a moisturl'ie content of about 0.2% and wound up on a storage ro (3) providing a liquid polymeric composition which has been prepared by (a) making a polymer solution consisting of 12.9% polyurethane elastomer, 4.3% polyvinyl chloride and 82.8% dimethylformamide according to the teaching of the first two paragraphs of Example 1 of U.S. Pat. 3,238,055 except for minor differences in proportion of ingredients which will be obvious to a skilled chemist, (b) converting the resulting solution to a substantially colloidal dispersion by gradually adding a 1:4 blend of water and dimethylformamide to the solution while stirring until the mixture has a water content of 4.0% and (c) adjusting the temperature of the resulting composition to 25 C.;

(4) combining the resulting two fabrics and the liquid polymeric composition into a composite sheet material which (a) contains the woven fabric resulting from step 1 1n superposed relation with the nonwoven fabric described in step 2, (b) has a thin layer of the liquid polymeric composition in interposed contact with the two fabrics, but not necessarily a continuous layer since the fabrics usually touch each other at numerous points, (c) has a 65 mil-thick layer of the liquid polymeric composition on the top surface of the woven fabric, (d) contains the liquid polymeric material within the pores of both fabric layers, (e) is prepared by placing the woven fabric in superposed contact with the nonwoven fabric to form a composite fabric by simultaneously passing both fabrics over a common roller, applying the liquid polymeric composition at 25 C. to the resulting composite fabric by means of a doctor knife coating apparatus in an amount sufiicient to end up with the above-mentioned 65 mil thick layer on the top surface, and subjecting the bottom Surface of the composite fabric to suction (10 inches of mercury) for /2 second by means of a vacuum pump, thereby causing the lower portion of the liquid polymer layer to impregnate the woven fabric and the upper portion of the nonwoven fabric while retaining a 65 mil wet layer thickness on top of the woven fabric;

(5) passing the resulting composite sheet material coating-side-up at a speed of 20 feet per minute through a rapidly moving air stream so that the air stream causes the upper most portion (a thin layer at the surface) of the liquid layer to coagulate, this being accomplished by (a) moving the sheet material horizontally towards a width of /2 inch, (c) mounting an 8 inch diameter exit 1 duct at both edges of the sheet material at a point located feet upstream from the roller so that the air stream can be removed at this point, and ((1) allowing the air stream to flow upstream across the coated surface for a distance of 10 feet in the manner indicated in the previous steps, the air stream having a velocity of 150 feet per minute, a temperature of 24 C. and a relative humidity of 37%;

(6) bathing the liquid polymeric composition of the resulting composite assembly by (a) immersing the assembly in a tank containing water at 18 C. for 9 minutes, (b) immersing the assembly in a tank containing water at 65 C. for 9 minutes, and (c) passing the sheet over a rotary vacuum drum provided with a supply of clean rinse water at a temperature of 80 0., thereby completing the coagulation of the polymer and the leaching therefrom of organic solvent; and

(7) drying the resulting 100 yards of composite sheet material for 5 minutes in an oven heated to 140 C.

Steps 5, 6 and 7 cause the liquid polymeric composition to solidify in situ and become a microporous moisture-permeable structure which forms not only a smooth, durable, adherent coating on the top surface of the sheet material but also a strong bonding layer which integrally adheres the two fabrics together.

Shoes in which the leather-like product of Example 1 is used as the upper material have excellent appearance, smoothness, durability and comfort properties. In the microporous coating, the pore size is very small and surprisingly uniform. The product has a substantially blemish-free surface, and the coating is easily dyed to a uniform color by known dyeing procedures.

EXAMPLE 2 An unsupported microporous film is prepared by (1) providing a nonwoven fabric as described in step 2 of Example 1;

(2) treating the nonwoven fabric so that the microporous film can "be removed (upon completion of the process) this being done by (a) continuously unwinding the fabric from the storage roll, (b) passing the fabric into a tank filled with water at a temperature of 24 C., and (c) passing the immersed fabric out of the tank and then between a pair of squeeze rolls whereby the water content of the fabric is reduced to 75% based on the combined weight of fabric and water;

(3) providing a liquid polymeric composition as described in step 3 of Example 1;

(4) coating the wet fabric provided in step 2 with the liquid polymeric composition at 25 C. provided in step 3 by means of a doctor knife coating apparatus in an amount sufficient to end up (at the end of this example) with a dry film thickness of 20 mils;

(5) passing the material through an air stream in accordance with step 5 of Example 1;

(6) bathing the layer of liquid polymeric composition in the manner described in step 6 of Example 1;

(7) drying the resulting 100 yards of material for 5 minutes in an oven heated to 140 C.; and

(8) stripping the 20 mil thick microporous film from the nonwoven fabric substrate by merely pulling the film away from the substrate.

The resulting unsupported film has a smooth substantially blemish-free surface; it can easily be dyed to a uniform color by known dyeing procedures. The product has a permeability value of 20,000 (grns./hr./ 100 m. at

21 C., RR, 3 hrs.) as determined by the test described by Kanagy and Vickers in the Journal of American Leather Chemists Association, 45, 211-242 (Apr. 19, 1950) The product is useful in such applications as filters and surgical bandages.

I claim:

1. A process for preparing a moisture-permeable sheet material which comprises (A) applying to a substrate a layer of a liquid polymeric composition which is comprised of a polymer dispersed in a volatile liquid comprising a volatile organic solvent, and which layer is capable of being solidified in situ to form a microporous structure when applied to a substrate and then subjected to steps B, C and D below,

(B) exposing said layer of liquid composition to an air stream having a velocity of at least 10 feet per minute and a relative humidity of at least 16% until said layer has absorbed enough water to cause at least the uppermost portion of the layer to coagulate,

(C) bathing said layer of composition with a volatile bathing liquid adapted to extract the volatile organic solvent from said composition until substantially all of said solvent is extracted, and

(D) drying the resulting sheet material, steps B, C and D causing said composition to solidify in situ and become a microporous moisture-permeable structure.

2. A process according to claim 1 wherein said substrate is a porous fibrous substrate, and the microporous structure resulting from step (D) forms an adherent coating on the substrate.

3. A process according to claim 2 wherein said substrate is a fabric.

4. A process according to claim 2 wherein the liquid polymeric composition is a substantially colloidal dispersion which has been prepared by adding nonsolvent to a polymer solution according to the method which comprises forming a polymeric solution having as essential constituen-ts a polymeric component and solvent for said polymeric component, said polymeric compo nent having a maximum elastic deformation strength, under the drying conditions of the drying step below of at least adding to said polymeric solution, at a temperature at which a colloidal dispersion will form, an inert liquid, which is a nonsolvent for said polymer but which is miscible with said solvent, until about the point at which a substantially colloidal polymeric dispersion is formed, said substantially colloidal dispersion having a viscosity greater than about 1 poise and a polymer concentration of greater than about 7% by weight;

coating said dispersion onto a substrate;

directly bathing the resultant coating, at .about the temperature at which the colloidal dispersion was formed, with a liquid which is a non-solvent for said polymeric component and miscible with said solvent to remove the solvent; and

drying the resulting product.

5. A process according to claim 4 wherein said nonsolvent is water, and said liquid polymeric composition has a water content of about 3.745% by weight before it is applied to the fabric.

6. A process according to claim 4 wherein said air stream has a velocity of about 50300 feet per minute.

7. A process according to claim 4 wherein said air stream has a velocity of about 100-200 feet per minute.

8. A process according to claim 4 wherein said fabric is a composite fabric composed of a woven fabric in superposed relation with a nonwoven fabric.

9. A process according to claim 4 wherein the tempera- 7 ture of said layer of composition at the beginning of step B is about 20-35" C. and the temperature of the air stream is about 0.5-5.0 lower than the temperature of said layer of composition.

10. A process according to claim 9 wherein the air stream has a relative humidity of about 3050%.

11. A process according to claim 4 wherein said bathing liquid is a liquid selected from the group consisting of water and mixtures of water and water-miscible volatile organic liquids, said liquid being a coagulant for said liquid polymeric composition.

12. A process for preparing a moisture-permeable leather-like sheet material which comprises (1) providing a woven fabric,

(2) providing a nonwoven fabric containing a solidified polymeric impregnant having a porous moisturepermeable structure,

(3) providing a liquid polymeric composition (a) which is comprised of a polymer dispersed in a volatile liquid comprising a volatile organic solvent and (b) which is capable of being solidified in situ to form a microporous structure when applied to a fabric substrate and then subjected to steps 5, 6 and 7 below,

(4) forming a composite assembly from the components resulting from steps 1-3 so that said woven fabric is in superposed relation with said nonwoven fabric and there is a layer of said liquid polymeric composition on the top surface of said assembly as well as in interposed contact with said fabrics,

(5) exposing at least the top surface of said assembly to an air stream having a velocity of about 100-200 feet per minute and a relative humidity of about 30-50% until said layer has absorbed enough water to cause at least the uppermost portion of said layer to coagulate,

(6) bathing the liquid polymeric composition of said composite assembly with a volatile liquid adapted to extract the volatile organic solvent from said composition until substantially all of said solvent is extracted, and

(7) drying the composite assembly; steps 5, 6 and 7 causing said composite to solidify in situ and become a microporous moisture-permeable structure which forms not only an adherent coating on the top surface of the resulting moisture-permeable composite article but also a bonding layer which strongly adheres said fabrics together.

13. A process according to claim 12 wherein step 4 is carried out by (a) placing the woven fabric in superposed contact with the nonwoven fabric to form a composite fabric,

(b) applying a layer of said liquid polymeric composition on the top surface of the composite fabric, and

(c) subjecting the bottom surface of the composite fabric to sub-atmospheric pressure until the lower portion of the liquid polymer layer applied in step (b) has flowed downward enough to fill at least part of the pores of both fabric layers and to form a layer of the polymeric composition in interposed contact with the fabric layers.

14. A moisture-permeable sheet material prepared according to the process defined in claim 1.

15. A process according to claim 1 wherein said substrate is a sheet material adapted to be removed from the microporous structure, and the microporous structure is removed from the substrate as an unsupported microporous film.

References Cited UNITED STATES PATENTS 3,067,483 12/1962 Hollowell 16164 3,100,721 8/1963 Holden 1l7-135.5 3,23 8,055 3/1966 Brightwell 11765.2 3,407,104 10/1968 Crandall 156285 X 3,418,198 12./1968 Einstman 156285 X CARL D. QUARFORTH, Primary Examiner G. G. SOLYST, Assistant Examiner US. Cl. X.R. 

